This invention relates to improved lithographic printing plates with a resin reinforced image and to a process for making such plates.
In the art of lithographic printing plates, there are many options for producing an image useful for printing. These include an additive, one-step resin reinforced image, a substractive, photo-polymer image, a substractive diazo-resin reinforced image, a substractive photo-polymer on diazo image, an additive, one-step reinforced image post cured with heat, a substractive diazo image, photo-polymer image reinforced and post exposed, bi-metal and trimetal plates, and resin particles in a diazo matrix.
A problem associated with these plate systems has been the durability of developed images. While offset lithography represents one of the most widely practiced of the printing arts, it is nevertheless generally limited to applications where relatively short press runs are acceptable. This is due principally to the abrasive action of the pigments employed in offset inks coupled with the physical interaction between the blanket cylinder and the plate master cylinder which results in relatively rapid wear of the olephilic image areas of the printing plate. Thus, conventional photolithography while highly desirable in many respects does not compete effectively with letterpress printing for large volume printing applications. Early efforts directed towards solving this problem involved reinforcing the image after it was developed by applying a durable coating in the image areas. However, such coatings had to be applied properly, skillfully and uniformly and failure to achieve any of these led to undesirable and often disastrous results. This prompted the development set forth in U.S. Pat. No. 3,136,637, Larson, June 9, 1964, of presensitized structures having a water-insoluble solvent-softenable polymer coating over the entire light-sensitive layer. After exposure to actinic light, the portions of the polymer coating overlying the soluble unexposed portions of the light-sensitive layer are removed with a suitable solvent and the soluble portions of the light-sensitive layer are removed with a second solvent which is generally water.
Another earlier approach involving coating the entire light-sensitive layer before imaging is set forth in U.S. Pat. No. 1,992,965, Rowell, Mar. 5, 1935. Here, however, a film of waxy material is applied in thicknesses of two or three ten thousandths of an inch to maintain and preserve the actinic sensitivity and surface continuity of chromated colloid films. This waxy film preservative, which is actually less durable than the chromated colloid film, is removed with a solvent after exposure to actinic light so that the colloid can be developed with water or is made water-permeable by laying down a wax emulsion and removing the water. In the latter instance, water passes through the waxy film and removes the soluble portions of the colloid film as well as the overlying portions of the waxy film. This is undesirable because it leaves less durable wax over the more durable image areas of the colloid film.
Later efforts involved a protective layer over the light-sensitive layer which is permeable to solvents used to develop an imaged plate. See U.S. Pat. No. 3,773,514, issued Nov. 20, 1973, and U.S. Pat. No. 3,839,037, issued Oct. 1, 1974. The protective layer is a very thin layer of metal or metal compound which is vacuum deposited. This was also used to improve vacuum metalized articles. See U.S. Pat. No. 3,775,157, issued Nov. 27, 1973.
One of the major problems in current lithographic plate systems is the difficulty in toughening the image area without resorting to toxic, complex solvent systems. For example: most additive systems resort to heterocyclic or aromatic solvents, some of which are dangerous both to the user and the environment. They are furthermore characterized by variable quality and run lengths, depending on the skills of the plate-maker. Most of these additive systems require the presence of strong acids, masking aids, and inorganic particulates.
Solvent systems incorporating resins are further limited by the molecular weight of the resin that can be put into solution, fifty thousand Daltons being generally the upper limit. Thus more durable, high molecular weight resins cannot be used. Conventional subtractive photo-polymer plates require relatively high energy inputs (e.g., on the order of 10.sup.7 ergs per cm.sup.2) to effect the necessary chemical reaction. Most of these systems incorporate complex organic solvent/water miscible developers which are often environmentally dangerous.
The present invention represents a marked departure from prior conventional plate making systems. The present invention provides an improved plate and process which is characterized by total aqueous development, quick, low energy imaging (e.g., on the order of 10.sup.4 ergs per cm.sup.2), and an extremely tough, long-wearing oleophilic image.